The emergence of cryogenic fuels such as liquid hydrogen and liquid oxygen as the leading candidates for use within orbital transfer vehicles and transatmospheric vehicles results in a need for reliable cryogenic fuel turbopumps. These turbopumps are placed intermediate the fuel tanks and the engine assembly in order to deliver cryogenic fuel to the engine at a desired high pressure. The turbopumps must operate at or near the cryogenic temperatures of the fuels while it is essential that the turbopumps be extremely reliable. Additionally, use on these types of vehicles inherently requires an efficient and compact configuration.
Generally, the cryogenic fuel turbopump will have a rotating assembly including a turbine and a pump mounted on a common shaft all contained within a housing assembly. Since the rotating group is the only moving element within the turbopump, mounting and support of the rotating assembly becomes the critical factor with respect to the reliability and life of the turbopump. Currently, cryogenic turbopumps use ball bearing type journal bearings to support the rotating assembly. However, ball bearings operating at cryogenic temperatures have short and unpredictable service lives. The premature failure of ball bearings in these turbopumps may be traced to one or more of the following factors. First, cryogenic temperatures preclude the use of conventional oil lubricating methods. Second, these turbopumps operate at high speeds for maximum efficiency, at these speeds the centrifugal forces on the balls become excessive and thereby limit bearing life. Third, to adapt ball bearings to high speed applications, the bearing diameter is reduced to achieve a lower DN (diameter times RPM) value, adversely resulting in the shaft bending critical speed approaching the maximum operating speed and making the rotating assembly sensitive to out of balance forces. Thus, ball bearing systems are not deemed practical for high reliability turbopumps that require long life.
As an alternative to ball bearings, the use of hydrostatic journal bearings and tilting pad journal bearings have been explored. In the case of hydrostatic bearings that utilize the high pressure available in the turbopump, rotor dynamic performance is critical. This requires careful design and consideration of the interaction of bearing pressure, stiffness, damping parameters, and rotor natural frequencies during transient speed operation. To accomplish a desirable balancing of the factors, the estimated bearing clearance for high-speed turbopump applications is approximately 0.0012 cm (0.0005 in). These close clearances present initial fabrication difficulties, as well as operational problems related to centrifugal and thermal differential growth, and dirt contamination over the entire range of operating speeds and temperatures.
For high pressure high speed turbopumps, the axial forces acting upon the rotating group become very large, requiring careful design of a thrust bearing to accommodate these large forces. As in the case of the journal bearings, ball bearing type thrust bearings are not practical due to the low temperatures and high speed.
It is thus apparent that there is a need for a turbopump which remains operational at cryogenic temperatures, and which features an efficient, reliable, and durable mounting and supporting configuration for the rotating group.